Silicon carbide (SiC) is a material that offers great potential for
power-electronics applications in high-reliability aerospace and
military systems. Compared to conventional silicon devices, SiC's
improved electron mobility and high-temperature capability together with
a high breakdown voltage appears to offer an ideal combination of
features for power circuits. But can SiC deliver on those promises?

Part of the strength of SiC as a semiconductor material is its
wider bandgap than that of conventional silicon. The larger bandgap and
higher critical electric field allows the material to operate at higher
voltages and with lower leakage currents. Gallium nitride provides a
possible wide-bandgap, high-mobility alternative--and promises key
advantages for RF electronics. But in high-power electronics, SiC has
the advantage of supporting device structures similar to those used in
silicon MOSFETs.

SiC power devices can be fabricated by depositing epitaxial layers
on a SiC substrate, whereas GaN calls for a supporting substrate of
silicon or sapphire. The homogeneous nature of SiC provides
opportunities for devices, where it is possible, to form a useful path
from the top to the bottom of the wafer in terms of both electron and
thermal conductivity.

Such vertical devices can offer very high power-handling capability
with low on-resistance through the formation of numerous parallel
transistor or diode structures. Because SiC can handle a higher critical
electric field, the drift zone of a vertical diode or transistor can be
made much thinner than that of a silicon device, leading to further
reductions in on-resistance and power losses.

Reverse Leakage Current

There are further advantages. In principle, SiC devices can not
only tolerate much higher temperatures than silicon but radiation
bombardment. As a result, SiC has the potential to support space
applications.

Although the electronic properties of SiC were first explored more
than a century ago, the material is still relatively uncommon in
production applications. A key question is whether SiC can handle
sustained high temperatures during operation and not experience
degradation over time that will reduce overall reliability. A key
advantage of high temperature operation is that it reduces the need for
cooling and allows usage in harsher environments. To evaluate
performance, we performed tests on a group of Schottky barrier diodes
mounted in two different types of package.

Due to the wide bandgap of the base material, the diodes exhibited
leakage currents of less than 4[micro]A at the beginning of the test.
Operated at a temperature of 225[degrees]C for a period of 8,000 hours,
the diodes not only demonstrated continued low leakage, the performance
improved during the first 2000 hours, reducing leakage by up to 50 per
cent. As well as leakage, thermal impedance showed consistent behaviour
over time.

A common belief of designers about SiC Schottky diodes is that they
exhibit practically zero recovery time. In order to maintain a junction,
some capacitance is essential, which will lead to the need for some
recovery time for excess charge to clear. However, this time is no more
than a third of that seen for silicon diodes. As a result, SiC diodes
can be expected to show efficiency gains in high-speed switching
applications. In addition, the reverse recovery performance of the
diodes showed almost no difference with temperature, from -55[degrees]C
to the peak testing temperature of 225[degrees]C.

Similar results were obtained for tests of the common configuration
of a high breakdown-voltage MOSFET co-packaged with an antiparallel
diode--an architecture useful for motor control and power conversion
circuits to improve switching performance. Breakdown voltage remained
well in excess of the target of 1200 V and on-resistance also remained
stable over the 2,000-hour test period. Similar to the situation with
leakage current in the diode tests, the zero-bias current improved
slightly after a short while and remained stable over the rest of the
test period--and at levels close to just 25 percent of the target value
of 400 [micro]A.

Improvements to device structures are expected to lead to further
increases in performance. Manufacturers are beginning to develop
transistors that employ a double-trench structure, an advance on the
earlier planar and single-trench design. The single trench structure
exhibits lower on-resistance than a pure planar design within a vertical
transistor architecture but the single trench leads to the formation of
a parasitic bipolar transistor.

A move to the double-trench architecture, which places the gate and
source in their own trenches, eliminates the parasitic element. The
structure further reduces on-resistance compared to the single-trench
design and, thanks to reductions in the electric field strength around
the gate, should deliver even higher reliability.

Further experiments have investigated SiC's radiation
hardness. Working with the European Space Agency and the Japanese
Aerospace Exploration Agency, tests have demonstrated that the
total-dose performance of SiC is excellent, achieving the minimum
requirement of 100Krad, with the potential to push resistance further.
For single-event upsets (SEUs), the indications are that circuit
designers will need to take account of possible spurious effects.
Performance is worse than that of conventional silicon devices but
improvements to device structure (double trench) are expected to yield
better resistance to SEUs.

To achieve optimum use of the high-temperature capabilities of SiC,
the designer needs to consider the impact of packaging. Few materials
can withstand as much heat as the semiconductor itself over long periods
of operation. However, tests have demonstrated that silicon nitride
provides an effective packaging material. Although aluminium nitride
exhibits better thermal conductivity, it is brittle and requires a
baseplate for mechanical stability. Silicon nitride, on the other hand,
is strong enough to be used without a baseplate, which reduces the
overall cost of the material when used for packaging.

The result of these tests, with further work ongoing, has increased
the confidence in SiC as a key material for high-efficiency power
electronics in systems that need high reliability. Thanks to these
efforts, SiC-based power modules are being integrated into aerospace and
military systems where size, weight and cost reductions have been made
to the overall system.

by Rob Coleman, Power and Hybrid Business Development Manager, TT
Electronics

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